Our object in the proposed research is to develop optical probes (fluorescent dyes and chromophores) that can be used to (1) measure membrane potentials in nerve tissue and cells that are too small for microelectrode studies, (2) measure intracellular pH, and (3) identify transformed cells and quantitate the antigenic makeup of cell surfaces. We also plan to determine the molecular mechanism by which these new probes carry out their functions. We intend to accomplish our objectives by careful design, synthesis characterization and evaluation of the new optical probes. Evaluation will be carried out with biological and model systems. Much effort will be directed toward development of the highly fluorescent and absorptive polymethine dyes as optical probes of biological systems.